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Full text of "Handbook Of Chemical Engineering - I"

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438                               CHEMICAL ENGINEERING
and the indicator or recorder. A common cold junction is also placed here. In the illustration this is located in a pipe buried 10 ft. underground. The selective switch and recorder or indicator are usually mounted in a single case. The common cold junction and the junction-box compensating couple are connected at the recorder between the switch and the binding-post terminals of the instrument as illustrated. The cold junction is placed near the junction box and the recorder or indicator (with switch if desired) may be any distance away since only copper leads are used from this point to the junction box. The method is especially useful where separate cold junctions require too long compensating leads. The following example illustrates a case where such an installation is desirable. Suppose the temperature of a coke oven, 20 by 40 by 150 ft. is measured by nine couples inserted in the top. The indicator is located at the ground level 40 ft. from the furnace. The buried cold junctions are 20 ft. in front of the oven. The following illustrates the amount of compensated lead wire required to reach the buried cold junctions when the junction box is not employed.
Three couples at rear of furnace..........  3(150 -f 20 -f 20 + 10) ft.
Three couples at center of furnace........    3(75 + 20 -f 20 -f 10) ft.
Three couples at front of furnace.........             3(20 -f 20 + 10) ft.
Total compensating cable..............                               1,125 ft.
Consider the same installation when a junction box is located on top of the furnace at the center.
Three couples at rear of furnace.............                           3(75) ft.
Three couples at center of furnace...........                           2(10) ft.
Three couples at front of furnace...........                            3(75) ft.
From box to cold junction.................   (75 -f 20 -f 20 -f 10) ft.
Total compensating cable................                              595 ft.
By means of the junction box we effect a saving of some 500 ft. in the compensating cable and need to bury only one pair of junctions, and just as satisfactory an installation is obtained. In installing a large multiple-couple equipment with a junction box, it is very important to insure that the common cold-junction couple is connected with the correct polarity as -llustrated. Although we have used a common cold junction for all couples we .have not employed the objectional common return. Individual returns are used with every couple shown in the diagram.
In case the recorder is placed where the temperature is quite uniform from day to day, the use of a buried cold junction, or thermostated cold-junction box is not absolutely essential. The electromotive force generated at the junction box in Fig. 14 is then compensated for by running one pair of compensating leads from the recorder to the junction box, taking care to connect the negative lead to the negative terminal of the recorder and the positive lead to the selective switch. A simple installation of this kind is illustrated by Fig. 15. Here only one couple is shown, but as many couples as desired may be connected to the multiple-pole selective switch. The compensating lead wires are soldered together inside the junction box. The auxiliary couple formed by the compensating leads is in series with the couple connected in by the selective switch. The cold junction is accordingly at the recorder where the temperature is fairly constant. Changes in temperature of the distributing or junction box thus will not affect the reading of any couple.
Determination of Temperature of Buried Cold Junction.—Several methods are available for obtaining the temperature at the bottom of the junction well. The simplest is to use a thermocouple consisting of the compensating